1. Field of the Invention
The invention relates to a process for the production of expanded materials based on polyurea elastomers.
2. Description of the Related Information
Because of their versatility and adaptability to production conditions and consumer requirements, polyurethanes have become established worldwide as important and indispensable components of industrial polymers for the production of expanded products. The production of polyurethanes is based on matured working methods.
For a wide variety of reasons, polyurethane technology is now having to meet new requirements which demand permanent changes with regard to production and processing conditions. Reference may be made here, in particular, to the avoidance of fluorochloro-hydrocarbon and other halogen based blowing agents. Although it is possible to change to substantially more expensive substitute products of the same type having less potential with regard to environmental pollution, the use of halogen-free blowing agents, carbon dioxide in particular, is preferable for ecological reasons and much more attractive simply for economic reasons. In spite of many experiments carried out in the past, the use of carbon dioxide as a substitute for fluorochloro-hydrocarbons has not proved suitable owing to frothing, as described in a very recent publication (cf. M. Taverna, Conference Paper UTECH 1990, pages 70 to 73).
Changes are essential in polyurethane technology for reasons other than the above-mentioned problems with the blowing agents used to date. Thus, for example, the product quality must be substantially improved, in order to dramatically increase the life of the molded materials. For example, the requirements with regard to permanent improvement in the load-bearing capacity and in the overall ageing characteristics of foams are of primary importance here. Also for reasons relating to quality, it is necessary to reduce odor and fogging problems with the finished products, both for the processing industry and for the consumers. Here, it should be noted in particular that all starting materials not chemically fixed in the polymer matrix contribute to odor and fogging problems unless they have extremely high vapor pressures. This also affects aspects of occupational hygiene, toxicology and ecology. Unreacted isocyanates, phthalates as solvents for certain additives, partly volatile organometallic compounds and certain amine catalysts should be mentioned here as examples of the problem indicated.
Avoiding volatile constitutents as far as possible during production and in the finished product serves not only to improve quality, but also improves occupational hygiene, toxicology and ecology. Public interest in this subject is rapidly resulting in higher standards with regard to foam technology.
Novel raw materials, with which several or all of the topical problems of foam technology can be solved are therefore being searched for worldwide. To the forefront of such considerations is, inter alia, the use of relatively high molecular weight polyols as starting materials for polyurethane foam technology.
In the recent technical literature, there is a recognizable trend towards a changeover to polyetheramines as reaction components in the preparation of polyurethane. In contrast to traditional polyols which have at least two free hydroxyl groups, the polyetheramines have radicals of primary or secondary aliphatic amines or primary aromatic amines instead of the free hydroxyl groups. R. D. Priester, R. D. Peffley and R. B. Turner, Proceedings of the SPI-32nd Technical Marketing Conference, San Francisco, 1989, described the use of amino polyols having secondary aliphatic or primary aromatic amino functions for the production of expanded polyureas having a low density. They state that these materials have substantial advantages over conventional polyurethane foams in several respects. They provide markedly increased load-bearing capacity, considerably improved ageing characteristics and, not least, an improvement in the flame characteristics.
Furthermore, EP-A 0 279 536 states that long-chain, aliphatic, oligofunctional secondary amines, together with polyisocyanates in the presence of customary catalysts and additives, can be processed to polyurea foams, although the latter are not further characterized.
This technique can fulfil several of the requirements mentioned at the outset and is therefore to be regarded as an advance. However, it did not make it possible to dispense with traditional additives, for example with tin catalysts and amine catalysts which contribute to odor and fogging problems. Moreover, another disadvantage which affects the technical usefulness of these procedures is evident to one skilled in the art. In fact, it has not been possible to date to prepare such polyetheramines by simple methods which meet economic and technical requirements. These disadvantages relate to raw material costs, yields and/or the chemical purity of the resulting products.
The preparation and use of product mixtures of polyetheramines predominantly containing secondary amino functions for the production of expanded polyureas have frequently been described in the literature. The derivatization of long-chain, aliphatic, oligofunctional, primary amines (U.S. Pat. No. 3,654,370), for example by alkoxylation, cyanoethylation, alcohol amination and reductive catalytic amination, has been proposed.
Another procedure envisages using short-chain, primary aliphatic amines with polyoxyalkylene compounds in a catalytic reaction, similar to the teaching of U.S. Pat. No. 3,654,370, for the preparation of long-chain, aliphatic, oligofunctional, secondary amines; cf. U.S. Pat. No. 4,904,705.
Various procedures have also been proposed for the preparation of aromatic polyetheramines, for example the reaction of commercial polyhydroxypolyethers with isatoic anhydride in one step or by multistage processes. In this context, for example, DE-A 2 948 419 describes the conversion of commercial polyhydroxypolyethers with aromatic diisocyanates into prepolymers, with subsequent hydrolysis of the remaining isocyanate groups, aminopolyols having terminal amino groups bonded to aromatic radicals being formed.
However, it is known in polymer chemistry that the product properties of polymers can be better controlled and influenced the purer the raw materials used. This also applies to the formation and the product properties of the polyurethane polymers discussed above. In the case considered, especially with regard to the reactive terminal groups, the avoidance of random distributions of differently substituted amines would therefore be particularly advantageous.
It is possible, although only at considerably increased cost, to prepare very pure, long-chain, aliphatic, oligofunctional, secondary amines (cf. DE-A 38 25 637). Such products are prepared by hydrogenating Schiff's bases, which are obtainable from ketones and oligofunctional primary polyoxypropyleneamines.
In fact, oligofunctional polyoxypropyleneamines having primary terminal amino groups, the use of which for the preparation of polyetheramines having secondary amino groups is described in DE-A 38 25 637, represent raw materials which are available on a large industrial scale and which are obtainable, for example, under the trade name JEFFAMINE.RTM. (Texaco). The molecular weights of these substances are in the range between 230 and 8,000; their amine functionality is between 1 and 3.
The production of these products, for example according to U.S. Pat. No. 3,654,370, can be carried out in one step from petrochemical mass products, namely dioxypropylenepolyols and ammonia.
The direct use of such products for the production of expanded polyureas of low density would thus be much more economical and for technical reasons therefore desirable, because these substances are distinguished by a particularly high degree of amination and high uniformity with regard to their terminal groups. Another advantage of such products is that it is possible completely to dispense with structural elements such as esters, urethanes or ethoxy radicals for the synthesis of these polyol structures.
However, the literature shows that, although rigid and resilient, "expanded" polyureas having a high density and excellent properties can be produced using aliphatic polyetheramines with primary amino functions, the literature also shows that the extremely high reactivity of these polyetheramines prevent the production of expanded polyureas of low density.
In the production of "expanded" polyureas with high densities (800 to 1300 kg/m.sup.3) with particular reference to the use of primary, oligofunctional, long-chain aliphatic amines, maximum reaction times of 2 to 3 seconds are observed, during which the liquid raw materials used solidify to give a solid material no longer capable of flow.
According to the teachings of EP-A 00 81 701 and of U.S. Pat. No. 4,269,945, blowing agents can be used in these processes. The object of this is to achieve an improvement in the product properties, namely in the surface quality of rigid and resilient moldings. By means of this measure, microcellular, "expanded" substances having high densities of more than 800 kg/m.sup.3 are obtained. Only the use of permanent, dried gases, in particular nitrogen or air, has proved suitable in this procedure; this procedure is generally referred to as "nucleation". Although the use and/or concomitant use of autogenously produced carbon dioxide is proposed in these works, there is, however, absolutely no practical indication as to how the very rapid reaction, in which the reaction mixture solidifies to a mass incapable of flow, can be reasonably matched with the comparatively very much slower catalytic reaction of water with isocyanates. Furthermore, U.S. Pat. No. 4,910,231, in which, inter alia, a polyetheramine having primary amino groups can be reacted with an excess of polyisocyanate and water with formation of rigid foam, provides no ideas in this respect.
Accordingly, the process of the invention leads to the production of polyurea foams from polyetheramines having primary amino groups and polyisocyanates, in which, in spite of the high reaction rates to be expected in the case of such systems, foams having a low density and advantageous properties, inter alia, with respect to freedom from odor, resilience, mould release properties, content of unconverted diisocyanates and the like, can be obtained.